High Availability

High Availability (HA) is a solution that uses the Session Clustering
infrastructure to provide availability and continuity of mission critical
business applications.

Session Clustering HA is an additional safety layer
for maintaining session information integrity in Web cluster environments.
HA ensures that sessions will be serviced in case of a single failure.

Session Clustering HA provides all the current Session Clustering functionality
and is an optional feature for environments that require High Availability.

Note:

As an additional functionality layer, running HA may
have a slight impact on performance in comparison to regular session clustering
response time.

How does HA operate?

The High Availability layer preserves session information when a server
fails. Each session is saved twice, once on the master (originating) Session
Clustering Daemon and one on the master's backup Session Clustering Daemon.
This means that in the event that a master server fails, requests are
re-routed to the backup Session Clustering Daemon. Once a request is re-routed
to the backup Session Clustering Daemon, the Backup Session Clustering
Daemon turns into a master Session Clustering Daemon and creates a new
backup Session Clustering Daemon. A backup Session Clustering Daemon is
chosen based on it activity locating the Session Clustering Daemon with
the least amount of sessions and open sockets. In the event that two servers
fail session information will be preserved in the backup and can be replicated
if requested in its lifetime.

Regarding response time, the HA layer will not impose more than a 10%
performance degradation over the existing session clustering solution
(number of session requests per second).

As mentioned earlier, all servers that need to share session information
have to be associated to a cluster in Session Clustering Daemon. The HA
layer is also capable of identifying when a fallen server has been recovered
and will automatically return the fallen server into service.